Chapter 17

• What is mass spectrometry?


  An analytical technique that uses mass-to-charge ratios and abundance of gas-phase ions to identify the amount and type of chemicals in a sample
• How to perform mass spectrometry on a sample? (LONG)


  1. A sample is inserted in the mass spectrometer

  
  

  2. The sample is vaporised and converted into a gas phase

  
  

  3. the sample is then ionised in avacuum sealedionizer, and one way of doing this is using an electron beam: this beam of electrons act as billiard balls knocking electrons in the sample away, creatingpositive ions

  
  

  4. The POSITIVE Ions then pass through a NEGATIVE electric field, accelerating them

  
  

  5. Along the ion's path, a magnet is used to deflect the ion onto the detector:

  heavier ions move slower and are HARDER to deflect, whereas lighter ions are faster and are EASIER to deflect ----> HENCE THE IONS of EACHISOTOPEARE DETECTED AT SEPARATE PLACES ON THE DETECTOR

  
  

  6. When the ions pass through the magnetic field generated by the magnet, and hit the detector at a certain location, the detector can measure theradiusof the curved path that the ion took.

  
  

  7. This radius can be used to calculate the mass/charge ratio for this ion. For a 1+ ions the m/z will be the relative isotopic mass.

  
  

  8. The mass spectrometer takes this data and produces amass spectrum(the graph). The m/z ratio is plotted on the x axis, and the percentage abundance on the y axis.
• What is the short description for what happens in a mass spectrometer?

  1) Organic compound is placed in the spectrometer

  
  

  2) It loses and electron and forms a positive ion called the MOLECULAR ION

  
  

  3) The mass spec detects the mass to charge ratio(m/z) of the molecular ion which gives the molecular mass of the compound
• Show an example of a potential ion formed when ethanol is placed in a mass spectrometer?

  CH3CH2CH2OH(g) ----> CH3CH2CH2OH+(g) + e-

  
  

  Here, the molecular ion has a mass to charge ratio of60
• How can you find the molecular mass from the mass spectrum of a compound?

  
  

  3) In the diagram shown, the molecular ion peak is at ____ and hence the molar mass of the compound is ____

  1) To find molecular mass? - identify the molecular ion peak

  
  

  2) This peak is a clear peak at the highest m/z value-i.e. the right-most peak

  
  

  3) 72

  
• What is the M+1 peak?

  
  

  Why does it exist?

  1) This is a small peak one unit after the M+ peak and exists in mass spectrum

  
  

  2) This is because about 1.1% of carbon present in a sample is C-13. This gives rise to a small proportion of molecules which have a higher m/z ratio and hence are 1 unit along from the M+ peak

  
  

  3) However since the proportion of heavier molecules is extremely small, the M+1 peak is very small
• What is fragmentation?

  
  

  Fragment ions cause...

  
  

  What is the difference between fragments and fragment ions?

  Fragmentation is when some molecular ions are broken down into smaller pieces known as fragments.

  
  

  The fragment ions cause the other peaks in a mass spectrum

  
  

  
  

  3) Fragment is a general term for the species made when a molecular ion breaks up - it includes radicals as well

  
  

  However, fragment ions are the specific ions made from the breakdown
• What is the result of the simplest fragmentation possible?

  
  

  What is detected by the spectrometer and what is not?

  1) The molecular ion will split into two species- the FRAGMENT ION and a RADICAL

  
  

  2) The mass spectrometer DETECTS fragment ions but does NOT DETECT radicals- hence radicals do not appear on a mass spectrum.
• Why can't radicals (formed from fragmentation) be detected by a mass spectrometer?

  This is because they areuncharged- hence they are not picked up
• The mass spectrum for each compound is...

  
  

  Hence spectrometry can be used to...


  1)uniqueas molecules will all fragment in slightly different ways depending on their structure

  
  

  2) identify molecules - since even though two ions have the samemolecular ionpeak, the fragment ions found may be different.
• The M+1 peak can be used to...

  
  

  What is the formula for this?

  
  

  How many carbon atoms in a compound that has an M peak of 45mm high and an M+1 peak at 3mm high?


  1) determine thenumber of CARBON ATOMSpresent in an organic compound

  
  

  2) (height of M+1 peak/ height of M peak) x 100

  
  

  3) 3/45 x 100 = 6.66 =7 carbon atoms
• Atoms in MOLECULES are joined together by ___________ bonds.

  These bonds possess ___________ __________ and

  vibrate naturallyand constantly about a _________ point. This vibration increases with _________ __________________. Hence, when bonds absorb _____________ ____________ (thermal radiation) it makes the the bondsstrech or bend MORE1) Covalent

  2 + 3) Potential energy

  4) Central

  4 + 5) Increasing temperature

  6) Infrared radiation (IR)
• What are the two possible types of vibrations?

  BEND and STRETCH
• What is a stretch vibration?


  1) This is arhythmic movementalong the line between the atoms so that thedistance between the two atomic centres increases

  
• What is a bend vibration?


  1) This is a continual movement which causes thebond angle to change- i.e. it will increase then decrease then increase then decrease continually.

  
• What does the amount a bond stretched or bends depend on?

  1)MASS of ATOMS- heavier atoms vibrate more slowly than lighter ones

  
  

  2)BOND STRENGTH- stronger bonds vibrate FASTER than weaker bonds
• What is the overarching principle which allows IR spectroscopy to work?

  
  

  Why is wavenumber used as the X-axis scale as opposed to frequency (As it should be)?

  1) Any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond, and the natural frequency of a bond is unique to itself

  
  

  2) Frequency values are VERY large and so wavenumber is used because it is convenient and CAN be used because it is proportional to frequency.
• What are the units for wavenumber?

  cm-1
• What is the full process of carrying out infrared spectroscopy? (4)


  1) Sample under investigation is placed inside anIR spectrometer

  
  

  2) A beam of IR radiation in the range200cm-1 to 4000 cm-1

  
  

  3) Moleculeabsorbs some of the IR frequencies, and the emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample

  
  

  4) The IR spectrometer is usually connected to acomputerthat plots a graph oftransmittance against wavenumber

  
• In our atmosphere, why is the longer wavelength IR emitted from the EARTH absorbed and not the IR from the SUN?

  
  

  Eventually the vibrating bonds in these molecules...

  1) This is because frequency oflonger wavelengthIR is the same as the natural frequency of the bonds inmethane, CO2 and other greenhouse gases

  
  

  2)re-emit this energyas radiation that increases the temperature of the atmosphere close to the Earth's surface -leading to global warming
• ANALYSING AN IR SPECTRUM:

  
  

  1) What are the dips in the graph called?

  
  

  2) How do we use the peaks to identify the molecule?

  
  

  3) How is the X-axis different from most graphs?

  
  

  4) What is the region beneath 1500 cm-1?

  1) PEAKS

  
  

  2) Each peakpoints to a specific bondin the molecule, and which molecule it is, depends on the wavenumber at which the peak occurs

  
  

  3) It starts at 4000 and goes to 200 (DESCENDING)

  
  

  4) This is thefingerprint region- A region which contains UNIQUE peaks which can be used to identify the particular molecule under investigation
• TIPS FOR SOLVING IR SPECTRA PROBLEMS:


  1)All organic compoundsproduce a characteristic peak between2850 cm-1 and 3100 cm-1, due to the

  C-H bond- THIS IS NOT TO BE CONFUSED WITH THEO-H peakin ALCOHOLS
• What will the IR spectrum of an alcohol look like?


  1) C-H peak between2850 cm-1 and 3100 cm-1

  
  

  2) O-H peak between3200 - 3600

  
  

  There would also be a C-O peak between 1000-1300
• What will the IR spectrum of an


  aldehyde/ketonelook like?1) C-H peak between2850 cm-1 and 3100 cm-1

  
  

  2) C=O peak between1630 cm-1 and 3100 cm-1
• What will the IR spectrum of a carboxylic acid look like?


  1) C-H peak between2850 cm-1 and 3100 cm-1

  
  

  2) O-H peak between2500 cm-1 and 3330 cm-1

  
  

  3) C=O peak between1630 cm-1 and 3100 cm-1

  
  

  Also a C-O peak in the fingerprint region, however not really a good idea to use this region for conclusive identification
• What are the applications of IR spectroscopy? (2)


  1) Can be used to identifypollutants- Remote sensors can identify and measure pollutants like CO and CO2 and other hydrocarbons - hence IR can be used to monitorlocalised pollution levels

  
  

  2) Can be used inbreathalysers- Where a beam of IR is passed through captured breath in the sample chamber. If ethanol is present in the breath, then IR will be absorbed by the bonds. - the more IR absorbed, the higher the reading, and hence themore ethanol in the breath
• What would be the typical sequence for the identification of an unknown organic compound?

  1)Elemental analysis- This is when you can determine the empirical formula of the compound in question frompercentage composition data

  
  

  2)Mass spectrometry- Used to determine the molecularmass of compoundby looking at the molecular ion peak, and use of fragment ion peaks to identify sections.

  
  

  3)Infrared spectroscopy- Use of absorption peaks from an infrared spectrum to identify bonds andfunctional groupspresent in the molecule
• How can you use all three techniques to determine a compound?


  1) Empirical formula and mass spectrum tells you the molecular formula of the compound

  
  

  2) Using IR spectroscopy, you can identify the functional groups and hence the actual name of the compound
• What is a mass spectrum?

  A record of molecular damage from electrons
• What happens to molecules when bombarded by electrons in mass spectrometry?

  They are destroyed and broken into smaller pieces
• How can the identity of the original substance be deduced from mass spectrometry data?

  By analyzing the fragments produced
• What occurs when high energy electrons bombard a molecule in a mass spectrometer?

  The molecule may lose an electron and become positively charged
• What does the dot (•) represent in mass spectrometry?

  An unpaired electron remaining after ionization
• What can happen to the molecular ion after it is formed?

  It can fragment further into new ions or radicals
• What are radicals in the context of mass spectrometry?

  Neutral species with an unpaired electron
• What additional process can occur alongside fragmentation in mass spectrometry?

  Re-arrangement of the molecular structure
• What does each peak in a mass spectrum represent?

  A particular fragment with a specific m/z value
• What does the highest peak in a mass spectrum usually indicate?

  It corresponds to the molecular ion
• What is the m/z value for the molecular ion of octane?

  114
• What does the peak at 29 in the mass spectrum of octane correspond to?

  It corresponds to the fragment C2H5+